Thermoelectric device and manufacturing process therefor

Abstract

The invention relates to a thermoelectric device including a pair of different conductors joined at one head end to form a thermocouple junction, the opposite tail ends being free. The conductors have a circular or elliptical section, are arranged side-by-side in proximity of the head end and are joined by laser welding. Several pairs of conductors may be connected together by joining also the tail ends of a conductor of a pair with the tail end of the different conductor of another pair to form a thermopile comprising thermocouple junctions in series. The invention further relates to a process of manufacturing the device.

Claims

1. A thermoelectric device, particularly a thermopile, comprising: at least a pair of different conductors composing a first and a second element (1, 2) and joined at one head end to form a thermocouple junction (12), opposite tail ends being free, wherein said conductors (1, 2) have a circular or elliptical section, are arranged side-by-side in proximity of the head end, and are joined by laser welding.

2. The thermoelectric device according to claim 1, wherein the thermoelectric device comprises a plurality of pairs of conductors (1, 2) connected together by joining the tail ends of one conductor of a pair (1) with the tail end of the different conductor of another pair (2) to form a thermopile comprising thermocouple junctions (12, 21) in series, and wherein said plurality of pairs of conductors (1, 2) forms a thermoelectric generator.

3. The thermoelectric device according to claim 2, wherein different conductors of contiguous pairs (1,2) are arranged side-by-side in proximity of the tail ends (21) and with the head ends and/or tail ends (101, 102) substantially aligned with each other and joined by the laser welding.

4. The thermoelectric device according to claim 2, wherein a joining area of the head and/or tail ends of the conductors is shaped as two semi-cylinders with a circular or elliptical base joined along a generating line (105) with a gap between said semi cylinders (205′, 205″) at least partially occupied by melting material resulting from the laser welding to form a substantially oval section junction.

5. The thermoelectric device according to claim 2, further comprising a containment enclosure (7, 100), and housing the conductors (1, 2), the containment enclosure having a closed head end (110) receiving the head ends (12) of the pairs of conductors (1, 2) and closed, at a side of the tail ends (21), by a cap-shaped terminal (120) provided with passages for at least two signal drawing conductors (130, 140), thus forming a thermopile, the conductors being covered by insulating material and arranged in side-by-side relation, the first (3) and at least the last conductor (4) of the series being output terminals of the thermoelectric device connected to said signal drawing conductors.

6. The thermoelectric device according to claim 5, wherein the containment enclosure (7) is a circular or elliptical section jacket, said thermocouples in series being spirally wound and being housed inside the containment enclosure.

7. The thermoelectric device according to claim 5, wherein the containment enclosure is filled with insulating material composed, as desired, of one of the following materials: ceramic, glass, epoxy glass, fiberglass, glass fiber, or a mixture thereof.

8. The thermoelectric device according to claim 5, wherein within the containment enclosure (7, 100) housing the thermoelectric generator there is provided a jacket made of an electrically insulating material composed of a layer of insulating cement applied on an inner wall of the containment enclosure (7, 100) or an insulating material covering said thermocouples shaped as a tube or sheet wrapped around said thermocouples or a combination thereof.

9. The thermoelectric device according to claim 1, further comprising a plurality of different elongated conductor elements (1, 2) forming first and second different adjacent elements that are joined together at opposite ends to form thermocouple junctions (21, 21), wherein the elongated conductor elements (1, 2) have a circular or elliptical section and are joined by laser welding without weld material.

10. A method of manufacturing thermopiles comprising: (a) providing two metal wires made of a different material with a round, circular or elliptical section, and for each wire performing the following treatments: (b) washing the wires; (c) straightening the wires; (d) oxidizing and cutting to size the two metal wires; (e) mating with each other metal elements obtained from the two wires in a sequence of metal elements side-by-side made of a different metal, alternately with each other; (f) laser welding ends of said metal elements for a given length of end portions; (g) winding to form a corrugated or zig-zag element; (h) performing a stabilization thermal treatment, which includes baking for stabilizing in a tubular or cylindrical shape the corrugated or zig-zag element; (i) providing a housing tube and insulating the housing tube; (j) inserting incoherent or fluid insulating material in the housing tube; (k) inserting the wound or zig-zag corrugated element stabilized with the cylindrical shape in the housing tube; (l) optionally, vibrating the housing tube with the insulating material and the corrugated element inserted and strengthening by a heat treatment of the insulating material; (m) preparing drawing conductors together with a fastening nut and pre-inserting into a closing terminal of the housing tube shaped as a cap or cup; (n) laser welding the drawing conductors to the corresponding metal elements; (o) insulating the welds with an insulating sheath; (p) filling the cup terminal with cement and fitting on the housing tube; and (q) drying the cement.

11. The method according to claim 10, wherein step (j) is performed according to the following steps in alternative or in combination with each other: (i1) lining an inner wall of the housing tube with a layer of high temperature cement and then drying the high temperature cement; (i2) lining the inner wall of the housing tube with mica-based insulating paper with a high temperature resistance.

12. The method according to claim 10, further comprising an end step (r) that comprise performing a functional test to check that no short circuits to ground of the metal elements have occurred and that the thermopile has a predetermined electrical resistance.

13. The method according to claim 10, further comprising a step of oxidizing the metal elements welded together and forming an element corrugated or with a zig-zag pattern following step (f) to weld the metal elements with each other.

14. The method according to claim 10, wherein the incoherent or fluid insulating material is composed of ceramic, glass, epoxy glass, fiberglass, glass fiber, a mixture thereof in granular form or mixed to a liquid carrier.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0106] Features of the invention and advantages deriving therefrom will be more clear from the following detailed description of the annexed figures, wherein:

[0107] FIG. 1 is a perspective view of a thermopile according to the invention;

[0108] FIG. 2 is a section according to a diametral plane parallel to the axis of the thermopile according to FIG. 1;

[0109] FIG. 3 is an exemplifying front view of one embodiment of the invention;

[0110] FIG. 4 is the perspective view of the detail “A” of the previous figure that is the junction region of the device;

[0111] FIG. 5 is a top view of the detail of FIG. 4;

[0112] FIG. 6 is a section view of a thermopile according to prior art;

[0113] FIG. 7 is a section view of a thermopile according to one embodiment of the invention.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

[0114] With reference to FIG. 1, a thermopile comprises a body composed of a tubular housing case 100 having a closed end 110 and that at the opposite end is closed by an end cap 120 which has a passageway for two output or drawing conductors 130 and 140 drawing the electric signal generated by the thermopile. Inside the tubular housing case 100 a thermoelectric generator 150 is housed which, as it will be disclosed below, is composed of a sequence of thermocouples. Such thermocouples are identical to each other and are interconnected in series. A first piece of an initial thermocouple of the sequence of thermocouples is laser welded to one of the two signal drawing conductors 130. An end element of the last thermocouple of the sequence of thermocouples is connected in turn to the second signal drawing conductor 140.

[0115] In order to allow signals with different potential difference to be drawn, it is possible to provide at least one further drawing conductor or several further drawing conductors (not shown) connected to different elements of the thermocouples provided in intermediate regions of the sequence of thermocouples.

[0116] According to a specific feature, the inner wall of the tubular case and/or that of the end cup or cap 120 closing it are lined with a layer of cement or the like forming an electrically insulating layer between said sequence of thermocouples and the inner wall of the housing case or tube.

[0117] Still according to an advantageous feature, the tubular housing case 100 is filled with glass material or the like that holds and makes integral with each other the thermoelectric generator 150, the case 100 and it also covers with a protection effect the elements composing the thermoelectric generator 150, with a particular reference to the protection of elements made of constantan.

[0118] Still according to another feature (not shown), it is also possible to provide, between the inner wall of the tubular housing case 100 and the thermoelectric generator 150 as well as the filling material, a jacket made of insulating material and resistant to high temperatures, such as for example a jacket in or made of insulating paper resistant to high temperatures, such as paper with a certain amount of mica.

[0119] The layer forming the jacket wrapping the thermoelectric generator 150 can be made in the form of a tube or in the form of a sheet wound around said thermoelectric generator 150.

[0120] Moreover such jacket can be provided as an alternative or in combination with the lining made of cement of the inner wall of the housing tube.

[0121] The filling material can be composed of glass, ceramic, epoxy glass, fiberglass, glass fiber or the like or mixtures of such materials.

[0122] The case 100 is made of a metal material.

[0123] FIG. 3 shows in an exemplificative manner an assembly of pairs of different elements 1, 2 joined at the opposite ends to form a sequence of junctions of thermocouples 12 connected in series. The used materials are those conventionally used in thermocouples according to prior art such as for example, Chrome/Aluminum, Chrome/Constantan, Iron/Constantan, Nickel-Chrome-Silicon/Nickel-Silicon, Platinum/Rhodium, Iridium/Rhodium, Nickel/Molybdenum, Chromel/Constantan, Chromel/Alumel.

[0124] Each pair of elements 1, 2 is welded at one end 12. By welding also the opposite end 21, being careful to keep the alternate sequence of conductors of type 1 and those of type 2, a thermopile is obtained that simply is an assembly of thermocouples, the free ends of which are interconnected in series. Generally, the first and last terminal of the series 3, 4 are the output terminals of the thermopile, at which ends the voltage generated by Seeback effect is detected and to which the respective drawing conductors 130 and 140 of FIGS. 1 and 2 are welded. However, it is possible to provide also one or more output terminals connected to intermediate thermocouples, thus drawing a signal whose voltage corresponds to the unit voltage generated by a thermocouple multiplied by the number interposed between the end conductor and the intermediate signal drawing conductor.

[0125] Conductors are electrically insulated, for example by subjecting them to high temperature in oxidative environment and welded at the ends such to form thermocouple junctions.

[0126] The oxidation process can take place in two phases, a first phase of oxidation of the metal wires before sectioning them for forming the individual metal elements and a second oxidation phase after the individual metal elements are welded together and after completing the sequence of thermocouples, that is the thermoelectric generator.

[0127] FIG. 4 shows the detail of the junctions. Elements 1, 2 that have a circular or elliptical cross-section, are put side by side and caused to mate along a generating line 105 in proximity of an end. To this end a surface 6 can be used upon which the head surfaces 101, 102 of the elements 1, 2 are caused to abut such to guarantee an axial alignment.

[0128] There is no preferred position for the transverse alignment since the cylindrical symmetry allows the same superimposition surface to be obtained whatever the angle position of the parallel axis elements is. This is particularly important since, in thermopiles according to prior art, terminals forming junctions are flattened and therefore they need to be well aligned with each other both in axial and transverse direction such to guarantee them to be properly superimposed.

[0129] Thus, it is possible to create structures even with misaligned elements that is elements not lying all on the same plane. For the purposes of realization of the thermopile this does not involve particular drawbacks since the final device comprises a holding jacket 7, typically with a circular cross-section, where the conductors 1, 2 are fitted after having been suitably compacted and spirally wound. Angle misalignments between the elements lead to an irregular filling of the jacket, as pointed out in FIG. 7 with respect to the case of flat conductors shown in FIG. 6, an aspect that does not affect the operation of the device and that even allows a higher number of elements to be compacted thus increasing sensitivity.

[0130] The process of manufacturing the thermopile is described in detail above in the introduction description.

[0131] FIGS. 6 and 7 clarify well the concepts of the invention. The use of rounded conductor elements instead of flat ones, besides allowing the rolling step (to obtain flat terminals) and the positioning step (to obtain the exact superimposition of the terminals) to be eliminated, leads to the further advantage of allowing more reliable and quality welding to be performed.

[0132] With reference to prior art, the welding between different metals of the thermocouples occurs by melting them, generally without weld material for example by electric spot welding, in proximity of the contact points. In the case of flat conductors, the welding area has a shape following the profile of the contacts namely a rectangular shape. Moreover the melting will mainly occur at the area of the welding electrodes and also for a quite limited penetration depth in the thickness of the material. The areas surrounding the areas coinciding with the welding electrodes will be held one against the other by a simple contact, but they will not have constraints resulting from melted material and forming a kind of alloy.

[0133] By using a laser welding, on the contrary, if the electrodes have a cylindrical shape, the melted material pours into the gaps between the cylinders 205′, 205″ filling them as shown in FIG. 5.

[0134] Therefore by using the particular circular symmetry of the conductors it is possible to create a welding area containing an amount of melting material higher than the case of flat conductors considerably increasing the quality of the junction also as regards consumption of the electrodes, since the dimension of the volume where the melting generates an alloy between the two metals of the elements welded together is volumetrically higher and therefore it promotes the electrical contact.

[0135] The junction region of the ends of the conductors takes the shape of two semi-cylinders with circular or elliptical base joined along a generating line 105 with a gap between said semi-cylinders 205′, 205″ at least partially occupied by melting material resulting from welding. Thus a junction with a substantially oval cross-section is formed that is with arc portions joined by straight or curved lines 205, 205′.

[0136] The maximum efficiency is obtained by laser welding allowing the heat to be precisely directed by generating a higher amount of melt alloy inside the area 205′, 205″ between the two cylinders 1,2.

[0137] The use of laser has also the advantage of not having to entrust insulation of metal elements in the welded condition thereof. Electric spot welding requires, at least in the welding area, that the contact between the two parts to be welded is electrically conductive. If metal elements would be subjected to oxidation before being welded, by using the electric spot welding techniques it would be necessary to remove the layer of oxide at least in the welding areas and therefore an additional processing step should be added, for example using the process, defined as ROS (Remove Oxidation Sandblast), for removing oxide near the ends to guarantee the electric continuity necessary to the welding process by brazing or electric spot welding.

[0138] In order to avoid the above in known processes manufacturing thermopiles, the oxidation for generating the electric insulation layer of metal elements is performed with the elements in the welded condition.

[0139] This, however, causes the drawback that the formation of the oxide layer is not perfect, particularly in areas adjacent to the welding areas, since the two metal elements are very close if not in contact with each other in such areas, therefore, the oxidation treatment is not perfectly efficacious. This results in a possible electrically conductive contact between metal elements also in areas directly adjacent to welding areas and generates an extension of the contact areas in regard to the length of the metal elements, which is the same as shortening thereof and therefore as a reduction in resistance.

[0140] Therefore, each thermocouple generates a lower potential difference and in order to reach output signals with predetermined potential differences, in known thermopiles it is necessary to provide a higher number of metal elements with respect to elements necessary according to the present invention, with a reduction of the overall mass of the thermopile, less costs and more rapid response time.

[0141] The device according to the present invention can be widely changed as regards construction and can have many uses depending on the number of elements and on the type of used materials. In the field of burners, for example it is possible to obtain a good sensitivity by using 20 to 30, typically 25, thermocouples made of nickel-chrome/constantan with conductors with diameter of 0.6 to 1.0, typically 0.8 mm.

[0142] By using rounded cross-section conductors, the manufacturing process is considerably simplified, eliminating the step of rolling at least one of the two conductors and therefore the cost of wires of metal material, as well as eliminating, in the assembly process, also the steps required to properly orienting the conductors when placing them side-by-side for welding in the contact areas at the ends.

[0143] Laser welding provides wire oxidation before the step connecting the metal elements with each other, guaranteeing an electric insulation for all the length of the metal elements up to the welding area. This guarantees an optimal exploitation of thermoelectric capacities and therefore a higher efficiency of the generator as regards the voltage generated by the individual thermocouples, therefore, with the output voltage being the same, the number of elements of the generator is smaller than that of thermocouples of prior art.

[0144] Moreover, the melting generated by the laser produces an alloy between the two metals of the two conductors in the mutual joining area that makes the connection highly resistant from the mechanical perspective and from electrical conductivity perspective.

[0145] This is promoted also by the fact that the circular shape generates two recesses collecting the melted material that, therefore, is produced in an amount higher than electric welding techniques or brazing techniques (where even a third material is provided).

[0146] In this process, the advantage is also that while for the case of electric welding the two elements to be connected must be in contact with each other, therefore, the mutual positioning requires a certain degree of attention, in the case of laser welding the two areas to be connected can also be not in contact with each other, but simply put side-by-side.

[0147] The electrical contact between two metal elements therefore is better and has a lower resistance in the junction area, while in the immediately adjacent areas, the better and more accurate electrical insulation by the oxidation of the metal elements before being welded guarantees the fact that there are no contact areas outside the welding areas that limit the voltage generated by the pairs of metal elements.

[0148] All the above without departing from the information principle disclosed above and claimed below.